This invention relates to coated type magentic recording media, and more particularly, to an improved magnetic powder composition with which such magnetic recording media are coated.
In prior art magnetic recording media, needle-like iron oxide has been generally used as powder magnetic material.
In these years, a variety of magnetic articles of manufacture have been commercially utilized including high performance audio cassette tape, video tape, computer tape, multi-coated tape, magnetic discs, floppy discs, magnetic cards, and the like as symbolically represented by the commercial success of home VTR units. All these new applications favor magnetic recording media having higher signal recording density. The use of conventional needle iron oxide cannot fully accommodate such high density magnetic recording. Active works have been addressed to the development of new magnetic materials having higher coercive force and higher magnetic flux density.
One class of these magnetic powder materials includes magnetic metal particulates. The particulate magnetic metals have been commercially utilized in high performance audio cassette tape, video tape, and various magnetic discs. However, these particulate magnetic metals have some problems to be overcome. One problem is the low wear resistance of magnetic metal. It is significantly deteriorated during service operation. Another problem is that magnetic metal is readily oxidized. Magnetic recording media having such a magnetic metal undergo a substantial reduction in magnetic flux density due to oxidation, resulting in an output loss.
When a magnetic recording medium having such a magnetic metal is operated across a magnetic head of Sendust or amorphous metal material, there often occurs so-called "seizing". That is, a lusterless discolored layer is formed on the head surface, the layer being of different color from the head material. It is believed that the seizing is a chemical and physical quality change of Sendust or amorphous metal material on the surface due to the sliding contact between the magnetic recording medium and the magnetic head.
It is believed effective in preventing the "seizing" phenomenon that the magnetic recording medium be abrasive to such an extent that the magnetic recording medium may successively wear out the magnetic head as it passes across the head.
One of well-known methods for imparting abrasive action to magnetic recording media is by adding abrasives, for example, finely divided Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3 to magnetic recording media.
Our experience indicates that more than about 3% by weight of abrasive must be added to magnetic powder in order to prevent the seizing. If non-magnetic abrasive is added in an amount as large as 10% by weight or more in the magnetic recording medium, then the resulting magnetic recording medium is reduced in saturation magnetic flux density, detracting from electromagnetic characteristics.
Another problem is associated with a process of forming a magnetic recording medium using magnetic metal particles. It is imperative that an oxide coating is formed on the surface of magnetic metal particles during handling. As the case may be, an oxide coating is intensionally formed.
Magnetic recording media using magnetic metal particles having an oxide coating formed on the surface thereof have the advantages that they do not undergo a reduction in magnetic flux density due to the influence of external environments, for example, temperature and humidity or a deterioration in properties due to rust formation in the magnetic layer. However, since the surface oxide coating provides an increased electrical resistance, the magnetic recording medium tends to be electrically charged at the surface so that foreign matter will adhere to the surface, causing dropouts. Further, electric charges upon exfoliating cause discharge noises. All these factors detract from tape performance.